Compound semiconductors have different levels of band gap energy and lattice constants according to the material composition. Therefore, a multi-junction photovoltaic cell is produced, by which the wavelength range of sunlight is divided among a plurality of photovoltaic cells so that the energy conversion efficiency is increased.
Presently, a typical example of a multi-junction photovoltaic cell is a triple-junction photovoltaic cell (1.9 eV/1.4 eV/0.67 eV) including Ge cell/Ga(In)As cell/GaInP cell using a lattice matching material, provided on a germanium (Ge) substrate having substantially the same lattice constant as that of gallium arsenide (GaAs).
The efficiency of a photovoltaic cell made of a compound semiconductor is approximately two times as high as that of a silicon (Si) photovoltaic cell. However, a photovoltaic cell made of a compound semiconductor has a high-cost substrate or a small-sized substrate, and is thus significantly more expensive than a silicon photovoltaic cell. Accordingly, a photovoltaic cell made of a compound semiconductor is used for special purposes, mainly for use in space.
Furthermore, recently, a concentrated photovoltaic cell is formed by combining an inexpensive condensing lens made of plastic and a small cell of a photovoltaic cell made of a compound semiconductor. Accordingly, the usage amount of an expensive compound semiconductor is reduced compared to a typical flat plate photovoltaic cell formed without using a condensing lens. Such a concentrated photovoltaic cell can be manufactured at a lower cost and is used practically as a photovoltaic cell for general purposes other than special purposes as described above.
However, the power generation cost of a photovoltaic cell still remains high, and therefore it is imperative to further reduce the cost. Thus, studies are being conducted to increase the energy conversion efficiency and to reduce the manufacturing cost.
As an example of reducing cost, studies are being conducted to produce a photovoltaic cell with a compound semiconductor on a Si substrate that costs less by approximately one digit and whose area can be made large (see, for example, Non-patent Document 1). However, a Si substrate and a photovoltaic cell made of a compound semiconductor have different lattice constants, and a dislocation can occur due to lattice relaxation. Thus, a buffer layer is provided between the Si substrate and the photovoltaic cell layer for relaxing the lattice constant (performing lattice relaxation), in order to cause the difference in the lattice constant to relax as much as possible in the buffer layer, and therefore reduce the dislocation in the compound semiconductor.
Furthermore, there is proposed a method of forming a photovoltaic cell layer on each of the Si substrate and the GaAs substrate, pasting these together by a direct bonding method and removing the GaAs substrate, and forming a double-junction photovoltaic cell on the Si substrate (see, for example, Non-patent Documents 2 and 3).
Furthermore, there is proposed a method of manufacturing a photovoltaic cell by a smart cut method which involves implanting H+ ions, etc., inside a semiconductor substrate and peeling off a thin-layer from the substrate starting from the part where ions have been implanted. After implanting the ions, a Si substrate is bonded together with a Ge substrate, a GaAs substrate, or an InP substrate via SiO2. Then, by a heating process, the Ge substrate, the GaAs substrate, or the InP substrate is peeled off, and a photovoltaic cell made of a compound semiconductor is formed on a template substrate constituted by a Ge layer, a GaAs layer, or an InP-layer provided on the Si substrate (see, for example, Non-patent Documents 4, 5, and 6).
However, by the above methods of manufacturing a photovoltaic cell made of a compound semiconductor, an expensive GaAs substrate or InP substrate is used, and therefore the photovoltaic cell made of a compound semiconductor cannot be manufactured at a low cost.
As described above, by conventional manufacturing methods, the photovoltaic cell made of a compound semiconductor cannot be manufactured at a low cost.    Non-patent Document 1: Yamaguchi et al, Proceedings of the 28th IEEE Photovoltaic Specialists Conference (2002), pp. 860-863    Non-patent Document 2: The Japan Society of Applied Physics Autumn proceedings, 2010, 15p-NC-4    Non-patent Document 3: The Japan Society of Applied Physics Spring proceedings, 2012, 17p-DP3-6    Non-patent Document 4: Appl. Phys. Lett. 92, 103503, (2008)    Non-patent Document 5: Proceedings of the IEEE 4th World Conference on Photovoltaic Energy Conversion (2006), pp. 776-779.    Non-patent Document 6: Appl. Phys. Lett. 91, 012108, (2007)    Patent Document 1: Japanese Laid-Open Patent Publication No. S61-219182    Patent Document 2: Japanese Laid-Open Patent Publication No. 2006-216896